Much of today's search for new oil and gas supplies is being conducted offshore. Offshore operations, however, are usually more expensive than onshore operations and require a much larger commitment of capital with no greater guarantee that oil or gas will be found at any particular site. In a typical operation, an exploratory well is drilled from a floating vessel at a site where geological and geophysical data indicate the possibility of finding oil and/or gas in commercially producible amounts. If significant reserves are located, an offshore platform may be erected from which a number of wells may be drilled and completed. In conducting operations from the platform, the wellhead may be located above the surface of the water with the production and completion equipment available on the platform. In this case, the operation of the well is not greatly different from the operation of a well at an onshore site.
However, in very deep waters or where local conditions and expected returns make bottom-supported platforms uneconomical, it is desirable, if not necessary, to provide a subsea system for drilling, completing and producing wells. Although much effort has gone into the design of subsea drilling and production installations, the various designs proposed heretofore have been found to have certain shortcomings. In some subsea production installations, the necessary production equipment is made an integral part of the drilling template. Thus, production equipment is committed to the underwater site prior to verifying the actual extent of the field to be developed. In other subsea production installations, the equipment is arranged on the template in such a manner that maintenance and repair of the system is very costly and difficult. This results from the fact that certain production equipment cannot be removed from the template without first removing other associated equipment.
For instance, U.S. Pat. No. 3,618,661 discloses a subsea production installation having a multiwell drilling template, a manifold module releasably secured on the template, and a plurality of production wellhead assemblies or Christmas trees operatively connected on top of the manifold module. In this arrangement, the Christmas trees are connected on top of the manifold module such that the manifold module cannot be removed without first removing the Christmas trees. This, of course, is very inconvenient and costly should the manifold module have to be removed, to be repaired or replaced.
Another approach is disclosed in U.S. Pat. No. 3,778,812 in which the subsea production system includes a multiwell drilling template completely equipped with all the necessary manifold piping and equipment for handling well production fluids, gas injection, and well maintenance. In this system, the production wells are drilled through the fully-equipped template; thus, production equipment is committed to a particular site without first verifying the extent of the commercially producible reserves. Therefore, should the particular drilling site turn out to have fewer commercial wells than anticipated, an unnecessary and expensive investment in production equipment has been made.
From the above, it can be seen that there exists a need in the art for a subsea drilling and production system that is relatively inexpensive to maintain and that does not commit production equipment to a particular drilling site prior to verifying the actual extent of the reserves at the site.